Many types of computing and/or communications devices are capable of providing a wide array of functions while the devices move from place to place. These devices will be generically referenced herein as “mobile devices.” For a variety of reasons, it may be advantageous for a mobile device to be able to determine its current location. For example, some mobile devices execute applications, such as calendars, reminders, navigation assistants, and communication tools having functionality that may be altered or enhanced depending upon the location of the mobile device.
Most outdoor positioning systems use a Global Positioning System (GPS), which computes a device's position by triangulating distance to a set of satellites. The GPS infrastructure includes a constellation of satellites and ground tracking stations and depends upon published updates of satellite ephemerides. In order to determine its position, a mobile device may include a GPS receiver having a radio for receiving GPS satellite transmissions as well as sufficient computation and storage resources to estimate its geo-referenced latitude, longitude, and elevation (LLE). The LLE of a mobile device may be more easily viewed in the context of a map. Because GPS functions well only in outdoor regions with substantial sky visibility, it is desirable to have alternatives for environments that are typically incompatible with GPS.